Fuel cell systems are known in the art. A fuel cell is an electrochemical device which reacts hydrogen and oxygen which is usually supplied from the air, to produce electricity and water. The basic process is highly efficient, and for those fuel cells fueled directly by hydrogen, pollution free. Further, since fuel cells can be assembled into stacks of various sizes, power systems have been developed to produce a wide range of electrical power outputs and thus can be employed in numerous industrial applications. The teachings of prior art patents, U.S. Pat. Nos. 6,030,718, and 6,096,449, are incorporated by reference herein.
A fuel cell produces an electromotive force by reacting fuel and oxygen at respective electrode interfaces which share a common electrolyte. In the case of a proton exchange membrane (PEM) type fuel cell, hydrogen gas is introduced at a first electrode where it reacts electrochemically in the presence of a catalyst to produce electrons and protons. The electrons are circulated from the first electrode to a second electrode through an electrical circuit connected between the electrodes. Further, the protons pass through a membrane of solid, polymerized electrolyte (a proton exchange membrane or PEM) to the second electrode. Simultaneously, an oxidant, such as oxygen gas, (or air), is introduced to the second electrode where the oxidant reacts electrochemically in the presence of the catalyst and is combined with the electrons from the electrical circuit and the protons (having come across the proton exchange membrane) thus forming water and completing the electrical circuit. The fuel-side electrode is designated the anode and the oxygen-side electrode is identified as the cathode. The external electric circuit conveys electrical current and can thus extract electrical power from the cell. The overall PEM fuel cell reaction produces electrical energy which is the sum of the separate half cell reactions occurring in the fuel cell less its internal losses.
Since a single PEM fuel cell produces a useful voltage of only about 0.45 to about 0.7 volts D.C. under a load, practical PEM fuel cell plants have been built from multiple cells stacked together such that they are electrically connected in series. In order to reduce the number of parts and to minimize costs, rigid supporting/conducting separator plates often fabricated from graphite or special metals have been utilized. This is often described as bipolar construction. More specifically, in these bipolar plates one side of the plate services the anode, and the other the cathode. Such an assembly of electrodes, membranes, and the bipolar plates are referred to as a stack. Practical stacks have heretofore consisted of twenty or more cells in order to produce the direct-current voltages necessary for efficient power conversion.
The economic advantages of designs based on stacks which utilize bipolar plates are compelling. However, this design has various disadvantages which have detracted from its usefulness. For example, if the performance of a single cell in a stack declines significantly or fails, the entire stack, which is held together in compression with tie bolts, must be taken out of service, disassembled, and repaired. In traditional fuel cell stack designs, the fuel and oxidant are directed by internal manifolds to the electrodes. Cooling for the stack is provided either by the reactants, natural convection radiation, and possibly supplemental cooling channels and/or cooling plates. Also included in the prior art stack designs are current collectors, cell-to-cell seals, insulation, piping, and various instrumentation for use in monitoring cell performance. The fuel cell stack, housing, and associated hardware make up the operational fuel cell plant. Such prior art designs are unduly large, cumbersome, and quite heavy. Any commercially useful PEM fuel cell designed in accordance with the prior art could not be manipulated by hand because of these characteristics.
Fuel cells are, as a general matter, relatively slow to respond to increased load demands. When a fuel cell is used in a power distribution system, loads may vary over time. At some times, there may be spikes in the load. Because a certain amount of time is normally required to start up a fuel cell, additional fuel cells or fuel cell subsystems cannot be instantaneously brought on-line to handle instantaneous spikes in the load. At the same time, a spike in the load that exceeds the capacity of an on-line fuel cell can potentially damage the fuel cell. Thus, fuel cell overcapacity may be provided in prior art systems in order to handle short temporary spikes in demand. This type of design is inefficient and wasteful.
Fuel cells have, from time to time, been used in conjunction with charge storage devices, such as batteries, which can provide a more instantaneous power supply for given application needs. In most instances, the direct current (DC) power which a fuel cell power system produces must be converted to alternating current (AC) for most applications. In this regard, an inverter is normally used to convert the fuel cells DC power to AC. As a general matter, inverters generally function within a specified DC input voltage range. In some previous applications, the fuel cell and charge storage device have been coupled to an inverter which functions at the optimal voltage of either the fuel cell or the charge storage devices. In this arrangement, the voltage of the fuel cell was raised or lowered as appropriate, to provide optimum functioning of the system. Still further, altering the voltage resulted in decreased efficiency by way of heat loss incumbent in the conversion process.
The present invention addresses many of the shortcomings attendant with the prior art practices. For example, previous prior art applications which provide both a fuel cell and a charge storage device in the arrangement discussed above, have been unduly complex and have experienced as noted above, decreased efficiency by way of heat losses caused by the lowering of the voltages generated by the fuel cell to make the fuel cell voltage match, as closely as possible, the voltage capacity of the charge storage devices used with same.
Further, designers have long sought after means by which current density in self-humidified PEM fuel cells can be enhanced while simultaneously not increasing the balance of plant requirements for these same devices.
Accordingly, an improved ion exchange membrane fuel cell,is described in combination with a method for controlling same which addresses the perceived shortcomings associated with the prior art designs and practices while avoiding the shortcomings individually associated therewith.
Attention is directed toward the following patents, which are incorporated herein by reference: U.S. Pat. No. 6,028,414 to Chouinard et al.; U.S. Pat. No. 5,916,699 to Thomas et al.; and U.S. Pat. No. 5,401,589 to Palmer et al. U.S. Pat. No. 5,401,589 to Palmer et al. discloses a rechargeable battery provided in parallel with a fuel cell electrical output together with appropriate charging, switching and control means so that a sudden increase in power demand can be met by both the fuel cell and battery working together and/or a sudden decrease in power demand may be met by charging of the battery.
U.S. Pat. No. 5,916,699 to Thomas et al. discloses an energy storage system including a first energy storage device, such as a secondary or rechargeable battery, and a second energy storage device, such as a capacitor, fuel cell or flywheel. The second energy storage device provides intermittent energy bursts to satisfy the power requirements of, for example, pulsed power communication devices.
U.S. Pat. No. 6,028,414 to Chouinard et al. discloses a fuel cell stand-by energy supply system incorporating storage battery(ies) for supplying electrical power, the battery(ies) being recharged by the fuel cell.